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Determination of physical property limits for the gas accumulation in tight sandstone reservoirs in the eastern Ordos Basin

CAO Qing;ZHAO Jingzhou;LIU Xinshe;HU Aiping;FAN Liyong;WANG Huaichang;School of Earth Sciences & Engineering,Xi'an Shiyou University;PetroChina Changqing Oilfield Company;  
To determinate physical properly limits of natural gas accumulation in Upper Paleozoic tight sand reservoirs in the eastern Ordos Basin,natural gas stress condition was analyzed in micro pore throat,and irreducible water film,buoyancy,capillary resistance and abnormal high pressure and hydropower were considered for the effect of natural gas accumulation.Taking the thickness of irreducible water film as the minimum pore throat radius of tight reservoirs,it could be calculated irreducible water film and determined the lower limit of pore throat radius is equal to 10 nm based on the irreducible water saturation which measured by NMR.The maximum pore throat radius was calculated by critical pore throat radius according to equilibrium state of buoyancy and capillary resistance.To determine gas column height of main gas reservoirs at different geologic epoch,the upper limit of pore throat radius was calculated by critical pore throat radius.According to the results of mercury injection experiment,a matching tendency of the median pore throat radius and physical parameters could be established,and petrophysical limit parameters could be calculated based on generating the upper and lower limits into the tendency.The calculation results of petrophysical limits were:For the Member 8 of Lower Shihezi Formation,lower petrophysical limits of reservoir are 4% and 0.1 mD(porosity and permeability),upper petrophysical limits are 13% and 1.8 mD at the end of Early Cretaceous,and 14% and 2.0 mD at present;For 3rd sub Member of the Member 2of Shanxi Formation,the corresponding values are,in order,2%and 0.01 mD,8%and 1.0 mD,9%and 1.4 mD;For Taiyuan Formation,these are,in order,3.5% and 0.02 mD,11% and 1.1 mD,12% and 2.0 mD.
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